1. Field
Example embodiments relate generally to semiconductor devices and, more particularly, to the fabrication of semiconductor devices.
2. Description of Related Art
In general, fabrication of a semiconductor device involves performing a photolithography process on a semiconductor substrate to form a structure on the semiconductor substrate. Specifically, the semiconductor substrate may have a material layer and a photoresist layer stacked thereon. The photolithography process may be performed by mounting the semiconductor substrate in a projection exposure system. The projection exposure system may include a light source, a condenser lens, a photomask, and a projection lens, which are sequentially disposed along a light path.
In the case where a photomask includes a lot of mask patterns extending in one direction, the projection exposure system may be used by interposing a dipole illumination system between the light source and the condenser lens. The dipole illumination system may allow light from the light source to be obliquely incident to the condenser lens, thereby contributing to limiting primary light diffracted by the photomask to the projection lens. Thus, the projection exposure system may properly transfer the mask patterns onto the photoresist layer using the dipole illumination system.
However, when a semiconductor substrate has a step difference, the projection exposure system may not be able to obtain good aerial images of the mask patterns of the photomask on the photoresist layer even with use of the dipole illumination system. This is because the photoresist layer has a top surface with different levels corresponding to upper and lower steps of the semiconductor substrate. In other words, the projection exposure system may have good depths of focus (DOFs) with respect to the mask patterns in only a portion of the photoresist layer corresponding to the upper step or the lower step of the semiconductor substrate.